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Plusle and Minun can echo Pichu (and >2 echoes in the same slot would have to be made a thing,) Shadow Mewtwo from Pokken can echo Mewtwo, and that's basically it as far as fighters already in the game go.
The only other Pokémon similar enough to and different enough from each other to be made echo fighters of each other would be
>the genies
>the Lake Trio
>Kyurem's Black and White formes
>Lugia and Shadow Lugia
>the elemental monkeys
>Volbeat and Illumise
>Silcoon and Cascoon
>the Alolan forms, minus Exeggutor
and Meowth is the only one on the whole list with anything close to a fighting chance of ever being a fighter in Smash.
Remember, folks, Doctor Freaking Mario is distinct enough to be a legit clone.
Something from a future gen might be made specifically so it can have an echo though
